CIS 115 Lecture 5

A storage location in memory (RAM) Holds data/information while the program is running These storage locations can be referred to by their

namesEvery variable has three properties:

Name - reference to the location - cannot be changed

Value - the information that is stored - can be changed during program execution, hence the name “variable”

Data Type - the type of information that can be stored - cannot be changed

You the programmer make up a name for the variable

Visual Basic associates that name with a location in the computer's RAM

The value currently associated with the variable is stored in that memory location

You simply use the name you chose when you need to access the value

Copy and store values entered by the user

Perform arithmetic manipulation on values

Test values to see if they meet a criteriaTemporarily hold and manipulate the

value of a control propertyHold data/information so that it can be

recalled for use at a later point in the code

Data type - Specifies type of data variable can store

Integer variables: Long, Integer, Short, Byte Floating-point variables: Single, Double Fixed decimal point variable: Decimal Boolean variables: True, False Character variable: Char Text variable: String The Object variable

Default data type assigned by Visual Basic Can store many different types of data Less efficient than other data types

Data type Prefix Size ValuesByte byt 1 bytepositive integer value from 0 to 255Short shr 2 byteinteger from –32,768 to +32,767Integer int 4 byteinteger from +/- 2,147,483,647Long lng 8 byteinteger from +/-

9,223,372,036,854,775,807

Single sng 4 byte single-precision, floating-point number

Double dbl 8 byte double-precision, floating-point number Decimal dec 16 byte number with up to 28 significant digits

Char chr 2 byte Any single characterBoolean bln 2 byte True or False

String str (4 byte) Text - Any number/combination of characters

Date dtm 8 byte 8 character date: #dd/mm/yyyy#Object obj (4 byte) An address that refers to an object

First character must be a letter or underscore

Must contain only letters, numbers, and underscores (no spaces, periods, etc.)

Can have up to 255 charactersCannot be a VB language keywordNaming Conventions

Should be meaningful Follow 3 char prefix style - 1st 3 letters in

lowercase to indicate the data type After that, capitalize the first letter of each

word Example: intTestScore

A variable declaration is a statement that creates a variable in memory

Syntax: Dim VariableName As DataType Dim (short for Dimension) - keyword VariableName - name used to refer to variable As - keyword DataType - one of many possible keywords to

indicate the type of value the variable will contain

Example: Dim intLength as Integer

A starting or initialization value may be specified with the Dim statement

Good practice to set an initial value unless assigning a value prior to using the variable

Syntax: Dim VariableName As DataType = Value

Just append " = value” to the Dim statement= 5 assigning a beginning value to the variable

Example: Dim intLength as Integer = 5

Variable MUST be declared prior to the code where they are used

Variable should be declared first in the procedure (style convention)

Declaring an initial value of the variable in the declaration statement is optional Refer to default values (next slide)

Data type Default (Initial) value

All numeric types Zero (0)Boolean FalseChar Binary 0String or Object EmptyDate 12:00 a.m. on January

1, 0001

Actual value/data/information Similar to a variable, but can NOT

change during the execution of a program.

Examples of Literals: Numeric: 5 ; 157 ; 195.38256

String: “Paul” ; “Hello!!!” ; “Jackson, AL 36545”

Char: ‘a’ ; ‘1’ ; ‘?’ ; ‘@’ Boolean: True ; False

Syntax: Const CONST_NAME As DataType = ValueLooks like a normal declaration except: Const used instead of Dim An initialization value is required By convention, entire name capitalized with

underscore characters to separate words

What – Indicates the part of the program where the variable can be used

When – From the variable declaration until the end of the code block (procedure, method, etc.) where it is declared Variable cannot be used before it is declared Variable declared within a code block is only

visible to statements within that code block▪ Called Local Variable

Can be declared at the beginning of the class code window (General Declarations section) and be available to all blocks▪ Called Form Level Variable

Variables that share the same scope cannot have the same name (same name ok if different scope)

What – Indicates the part of the program where the variable exists in memory

When – From the beginning of the code block (procedure, method, etc.) where it is declared until the end of that code block When the code block begins the space is

created to hold the local variables▪ Memory is allocated from the operating system

When the code block ends the local variables are destroyed▪ Memory is given back to the operating system

Syntax: variablename = expression

Assigns the value of the expression to the variable. (The variable must be on the left and the expression on the right.)

Example: intNumber1 = 4 intNumber2 = 3 * (2 + 2) intNumber3 = intNumber1 IntNumber1 = intNumber1 + 6

A value of one data type can be assigned to a variable of a different type An implicit type conversion is an attempt to

automatically convert to the receiving variable’s data type

A widening conversion suffers no loss of data Converting an integer to a single Dim sngNumber as Single = 5

A narrowing conversion may lose data Converting a decimal to an integer Dim intCount = 12.2 ‘intCount becomes 12

VB provides a set of functions that perform data type conversions

These functions will accept a literal, variable name, or arithmetic expression

The following narrowing conversions require an explicit type conversion Double to Single Single to Integer Long to Integer

Boolean, Date, Object, String, and numeric types represent different sorts of values and require conversion functions as well

The Val function is a more forgiving means of performing string to numeric conversions

Uses the form Val(string) If the initial characters form a

numeric value, the Val function will return that

Otherwise, it will return a value of zero

Val Function Value Returned Val("34.90“) 34.9 Val("86abc“) 86 Val("$24.95“) 0 Val("3,789“) 3 Val("“) 0 Val("x29“) 0 Val("47%“) 47 Val("Geraldine“) 0

Returns a string representation of the value in the variable calling the method

Every VB data type has a ToString method

Uses the form VariableName.ToString For example

Dim number as Integer = 123lblNumber.text = number.ToString

Assigns the string “123” to the text property of the lblNumber control

Arithmetic Operators^ Exponential* Multiplication/ Floating Point Division\ Integer DivisionMOD Modulus (remainder from division)+ Addition– Subtraction& String Concatenation

(putting them together)

Examples of use: decTotal = decPrice + decTax decNetPrice = decPrice - decDiscount dblArea = dblLength * dblWidth sngAverage = sngTotal / intItems dblCube = dblSide ^ 3

Concatenate: connect strings together Concatenation operator: the ampersand (&) Include a space before and after the & operator Numbers after & operator are converted to strings How to concatenate character strings

strFName = "Bob" strLName = "Smith" strName = strFName & " “ “Bob ” strName = strName & strLName

“Bob Smith”

intX = 1 intY = 2 intResult = intX + intY strOutput = intX & “ + “ & intY & “ = “ & intResult “1 +

2 = 3”

ParenthesisExponentialMultiplication / DivisionInteger DivisionMODAddition / SubtractionString ConcatenationRelational Operators (< , > , >= , <= ,

<>)Logical Operators (AND, OR, NOT)

6 * 2 ^ 3 + 4 / 2 = 507 * 4 / 2 – 6 = 85 * (4 + 3) – 15 Mod 2 = 34

intX = 10 intY = 5 intResultA = intX + intY * 5 'iResultA is 35 iResultB = (intX + intY) * 5 'iResultB is 75 dResultA = intX - intY * 5 'dResultA is -15 dResultB = (intX - intY) * 5 'dResultB is 25